Just out of the blue one day mail started to get stuck in the "messages pending submission" folder in the queue.

The only thing that we have change recently is that we upgraded our AD server to WS 2003 Ent that also has Dns and DHCP running on it but this was about a 2 weeks before this problem began.

When we first encountered the problem we restarted the server and a couple of messages where released but then got stuck again. Since doing research on this we found that if we end the inetinfo.exe process the mail will start to send and will continue to send for a fair few hours before getting stuck.

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Alot of the times when messages get stuck in the submission queue it's due to third party apps or event sinks that process the message before it's given to the transport mechanism. You can try disabling your AV\SPAM stuff, restart SMTP and possibly your Exchange services to test. Did you make any changes or updates to your SPAM app?

We have also seen the posts about the Trend Mirco problem and i have since excluded the recomended files from the virus protection. This has not totaly fixed the problem though but seemed to make it less frequent. No changes where made to the virus protection or spam protection before this problem occured.

I will try shutting down spam protection for a while see if that makes any diffrence.